Phase-lock systems for variably moving one signal in frequency/phase until it matches or "locks" with another reference signal are well known. When such systems are fabricated on chips, they often require a manual selection of several circuit elements that are external to the chip system itself. For example, in all systems an external de-coupling capacitor is normally used, and that external element is also required in the system of this invention. In addition to that capacitor, however, several other external elements are normally required in the prior art systems. Both the selection and several additional external elements are serious drawbacks to these prior art systems.
The system approach of this invention achieves a phase lock selection automatically. A major system component is a ring oscillator of selectable length and variable delay. The ring oscillator includes a plurality of tandem-connected current-starved inverter stages, each forming one curve in the system's family of curves. Automatic frequency/phase selection determines the proper operating point on the proper frequency curve without any extra external elements as required by the prior art.
A concurrently filed application, entitled CURRENT STARVED INVERTER VOLTAGE CONTROLLED OSCILLATOR by one of the inventors hereof, describes two prior art conventional oscillator design approaches explained in articles entitled Design of PLL-Based Clock Circuits, by Deog-Kyoon Jeon et al published in the IEEE Journal of Solid State Circuits, Vol. sc-22, No. 2, April 1987, pages 255 through 261 and A Variable Delay Line PLL for CPU-Coprocessor Synchronization, by Mark Johnson and Edwin Hudson, published in the IEEE Journal of Solid State Circuits on Oct. 18, 1988 at pages 1218 through 1223. A current starved inverter, and a bipolar multivibrator varying a load capacitance are featured, and the designers, when several cells were cascaded together, elected to go with a shunt capacitor delay cell due to the severe non-linearity of the current starved approach.
In spite of the above-noted deficiencies of conventional current starved inverter approaches, however, the inventors of this application have persisted and have provided a broadband, low power, and highly noise free voltage controlled oscillator circuit based upon a current starved inverter cell with a troublesome high gain non-linear area eliminated by an anti-high-gain circuit. Each inverter cell defines one essentially linear frequency curve in the system's family of curves. Combining a series of the improved low gain cells in a variable length, variable delay ring oscillator with an automatic selection method, provides a broadband low gain system which was not heretofore thought possible in the art.